Combustion air preheaters are well known in the art, examples of which are in U.S. Pat. No. 1,927,215, and in the prior art cited in U.S. Patent Application Ser. No. 205,750, filed Nov. 10, 1980, now abandoned.
It is known that flue gases have a dew point at which the first drop of liquid forms in the gas. The liquid in the flue gases is primarily water. When the liquid forms, it absorbs carbon dioxide which in turn forms H.sub.2 CO.sub.3 which is very corrosive. Further, almost all fuels have some sulfur so that when the sulfur burns it forms sulfur dioxide and sulfur trioxide, which together with water form sulfurous acid and sulfuric acid. It is also known that the dew point of flue gases is affected by the amount of the sulfur trioxide which is present so that if enough sulfur trioxide is present, the dew point is reached at a much higher temperature, which is above the boiling point of water, which means that higher temperatures must be reached with the combustion or flue gases to prevent the formation of the liquid droplets. The liquid droplets formed at or near the dew point are relatively small, some as small as one micron. However, if the temperature of a vapor falls or is lowered a significant amount below its dew point, or if the vapor contacts a material such as metal that is at a temperature significantly below the vapor dew point, liquid will form at an accelerated rate, causing the liquid droplets to increase in size. If the droplet size is sufficient, then may "rain" or attach to the metal and roll down the side of the metal, such as preheater tubes, on which the droplets are formed. When that occurs, the liquid cannot be removed by the gas flowing over or by the liquid, resulting in a corrosive film on the metal. Because of such corrosive conditions which have been prevalent in the past, expensive metals such as stainless steel have been used in preheaters.
In some situations where preheaters have been used, the ambient temperature of the inlet air to the preheater is so cold that it causes vapors inside of the preheater tubes to condense out because the heat supplied to the preheater from the combustion or flue gases is not enough to heat the metal of the preheater tubes to a temperature above the dew point of the vapor in the tubes. The same situation may occur if the combustion chamber is operated in a low turn-down condition, such that the unit operates as if oversized, i.e. below its rated capacity.
Thus, the preheater itself may be contributing to, or causing, the formation of liquid droplets in the interior of the tubes because of the heat which is transferred from the combustion gas through the preheater tubes to the preheated air, which causes an increase in the temperature of the preheated air, but a consequent decrease in the temperature of the combustion gas in the preheater P.
One of the problems with the prior art is that it has failed to recognize that the size of the droplets can be controlled so that they are small enough for the combustion or flue gas to flow through the preheater at sufficient velocities so long as the flow path is substantially straight to prevent an accumulation of corrosion liquid in the preheater, thereby making it possible to use relatively inexpensive ordinary low carbon steel in the preheater. When changes in direction of the combustion gases such as disclosed in U.S. Pat. No. 2,665,840 occur, they create a centrifuge effect or a dead zone which promotes coalescence of the droplets. When the droplets attach to each other in a dead zone, corrosion occurs.